1. Field of the Invention
The present invention relates to an electronic apparatus incorporating battery such as a handy terminal, key-less entry device, ID tag and remote control device driven by battery.
2. Description of the Related Art
Now a conventional battery-driven electronic apparatus incorporating battery is described as follows. The FIG. 4 is a wiring diagram illustrating a conventional electronic apparatus incorporating battery driven by battery.
Referring to FIG. 4, an battery-driven electronic apparatus incorporating battery is comprised, for example, of a driven circuit 1 consisting of a receiver circuit 1a and a transmitter circuit 1b, a source battery 2 (for example, a 3V lithium battery), a capacitor 3 provided in parallel with the source battery 2, detecting terminals 4a and 4b, a resistor 6' (for example, of 200 ohm) disposed between an end of the source battery 2 and an end of the detecting terminals 4a and 4b.
The detecting terminals 4a and 4b are provided to enable to identify the voltage of the source battery 2 from outside without removing the source battery 2 from the electronic apparatus incorporating battery.
In the electronic apparatus incorporating battery with the above mentioned construction, an electric current in the order of several .mu.A is supplied to the drive circuit 100 by the voltage of the source battery 2 during stand-by mode but when it is turned into a predetermined operational mode, the drive circuit 100 drives the driven circuit 1 such as the receiver circuit 1a or the transmitter circuit 1b to turn it into an operation mode by supplying current in the order of several tens of mA from the source battery 2 to the driven circuit 1 to perform the predetermined operation.
By the way, the internal resistance of the source battery (for example, an alkaline battery or a lithium battery) generally increases as the voltage drops when the battery is exhausted. As the internal resistance increases, the value of the voltage to be measured will be varied due to the current of the source battery used in a circuit for measuring, so that if measurement is conducted with a minimal current without measuring the voltage by supplying the same amount of current as supplied in the operation mode, there has been a problem that it is impossible to confirm whether the apparatus will work without fail or not, when the apparatus is required to conduct a predetermined operation. Accordingly, in the prior art, the following identification circuit for the battery voltage has been utilized for measuring the battery voltage.
FIG. 5 is a drawing illustrating a conventional circuit for identifying battery voltage, which is comprised of, in the drawing, checking terminals 10a and 10b disposed so as to be connected to the detecting terminals 4a and 4b of the electronic apparatus incorporating battery, a resistor R1 for generating voltage drop by causing a current similar to one in the operation mode (for example, an average current to be consumed in the driven circuit 1) to flow in the source battery 2 of the electronic apparatus incorporating battery, an A/D converter 14 for converting an analog voltage value measured by the resistor R1 to digital signals and an identification circuit 15' for identifying the life of the source battery 2 by the digital signals from the A/D converter 14.
The conventional circuit for identifying battery voltage with above mentioned construction operates to cause a current of several tens of mA, or approximately the same value as that of the operation mode to flow through the resistor RI from the source battery 2 of the electronic apparatus incorporating battery by connecting the checking terminals 10a and 10b to the detecting terminals 4a and 4b of the electronic apparatus incorporating battery. It is adapted to provide the voltage generated in the resistor R1 into the identification circuit 15' via A/D converter 14 to display the result of identification of measured life of the source battery 2 by means of a display device not shown in the drawing.
However, in the above described prior art example, there was a problem that as the same extent of current as used in the operation mode is applied for voltage measurement, it is possible to measure correct voltage but the measurement would consume the battery. Further, also, if the detecting terminals 14 are short-circuited intentionally or accidentally, there occurs such problem as the voltage of the battery 2 will drop in a short while to cause the battery life to be shortened.
Especially in the case of an electronic apparatus incorporating battery which is not activated by the instruction given by an operator, such as an ID tag which turns its mode into the operation mode by receiving electromagnetic waves to conduct a predetermined operation, it is a big problem that it does not get into the predetermined operation.